14 June, 2011
- These vulnerabilities affect: Most current versions of Excel, which ships with Microsoft Office
- How an attacker exploits it: By enticing one of your users to open a malicious Excel document
- Impact: In the worst case, an attacker executes code on your user’s computer, gaining complete control of it
- What to do: Install Microsoft Office updates as soon as possible, or let Microsoft’s automatic update do it for you
As part of today’s Patch Day, Microsoft released a security bulletin describing eight vulnerabilities found in Excel — part of Microsoft Office for Windows and Mac. The flaws also affect some of the Office document viewer and converter applications
Though the eight vulnerabilities differ technically, they share the same scope and impact. If an attacker can entice one of your users into downloading and opening a maliciously crafted Excel document, he can exploit any of these vulnerabilities to execute code on a victim’s computer, usually inheriting that user’s level of privileges and permissions. If your user has local administrative privileges, the attacker gains full control of the user’s machine.
Microsoft has released patches for Office to correct all of these vulnerabilities. You should download, test, and deploy the appropriate patches throughout your network immediately, or let the Microsoft Automatic Update feature do it for you.
Excel update for:
For All WatchGuard Users:
While you can configure certain WatchGuard Firebox models to block Microsoft Excel documents, some organizations need to allow them in order to conduct business. Therefore, these patches are your best recourse.
If you want to block Excel documents, follow the links below for video instructions on using your Firebox proxy’s content blocking features by file extensions (.xls and .xlsx). Keep in mind, blocking files by extension blocks both malicious and legitimate documents.
- Firebox X Edge running 10.x
- Firebox X Core and X Peak running Fireware 10.x
Microsoft has released Office updates to fix these vulnerabilities.
This alert was researched and written by Corey Nachreiner, CISSP.
Alexander Skordos says
Well, my thought was, if the general policy of the system is strict meaning only necessary ports are opened outgoing and none incoming, but alllows excel files, IPS and A/V is enabled and scans, you can still get it?
Would Win7 UAC help in this case?
Corey Nachreiner says
Since most ppl allow http and email, uses often can download excel files, so can certainly succumb to this issue. That said, if you have our XTM appliance’s GAV enable, it can, and does, catch malicious Office documents… However, AV is never perfect, whether or not it would catch the malicious document really depends on the variant of malware/exploit embedding… That’s why I still think patching is the best recourse…. or you can use our HTTP and SMTP proxies to block all Excel docs, but that is too restrictive for some networks…
As far as Windows UAC…. yes, this can help mitigate malware to some extent. However, there are many bad things malware can do without superuser/administrator privilege. If the malware tries to open listening ports, or send raw TCP/IP packets, then the lack of administrative access can restrict it.. However, the malware can still have access to all your files….